in

(Note: Some types of cells undergo division by a process known
as amitosis, which Coes not sencern us here.)

REFERENCES: *Storey--Principles of Hygiene, Chaps. 5 and 4,

*Burlingame and Martin--General Siology, Chap. 2.
Dorsey--Why We Behave Like Human 3eings, Pp. 1-54.
Guyer--Being Wel’-Born, Chap. 3%,

Conklin--Tieredity and Envivonnent, Pp. 6-32 (1922 ed. )

*Kirkpatrick and Huettner~<Fundamentals of Health,Pp. 16-33.

* Required reading.

III. THs NEW INDIVIDUAL

Cell division and multiplication becomes of more immediate interest when its
relationship to the production of a new human veing is seen. Every individual
begins his existence as a single cell. This cz sll is formed by the union of a
haploid germ cell from the father and one from the mother, a process designated
as fertilization,

Ae

 

FiuRTILIZATION

The spermatozoon (a mature haploid male germ cell) meets and fuses with an
ovum (a mature haploid female germ cell). In this fusion of sperm and ovum
a new individual comes into being, or, to word it differently, conception
takes place. It oceurs in the Fallopian tube leading from an ovary to the
uterus (womb).

Pregnancy is established if this fertilized ovum passes along the Fallopian
tube and attaches itself to the wall of the uterus.

CLEAVAGE OF THE EGG

The process of mitosis begins in this fertilized ovum and cell division
proceeds with great rapidity. This is the commencement of a long serics of
such divisions, lasting the 280 days of pregnancy and resulting in an
increase of five million per cent.

Continued division of cells results in the formation, first, vf a solid sphere
which rapidly develops cavities in its interior. By means of infoldings

and buddings of the cell layers, the growing embryo taxes shape. Evolution
of the race is recapitulated in the course of embryogenic development,

OUTLINE OF HUMAN EMBRYOLOGY
le Passage of dividing egg from Fallopian tube into uterus.
2. Implantation of beginning embryo in uterine wall.

3. Formation of chorion, an outer layer of cells arising from a portion of
the egg. This structure becomes highly vascular and connects witn the
circulatory system of the growing embryo. There is no direct connection
between the blood vessels of the m ther and those of the embryo, but an
ehchange of nutrient materials from the mother and of wastes from the
embryo takes place through projections on the surface of the chorion.